It is known for conventional ultrasound transducers to include a dematching layer on the backside of an acoustic layer including one or more transducer elements. The dematching layer typically includes a material with a higher acoustic impedance than the acoustic layer. Using a dematching layer enables the ultrasound transducer to use a thinner acoustic layer to achieve the same resonant frequency as would be realized using a thicker acoustic layer. Using a thinner acoustic layer enables the acoustic layer to have a better electrical impedance match with the imaging system and helps to improve the sensitivity needed for a transducer of a given frequency.
It is generally desirable to design ultrasonic transducers to have as broad of an overall bandwidth as possible. One known way to achieve a broader bandwidth involves machining the active acoustic layer, typically piezoceramic (PZT), piezoelectric single crystal (e.g. PMN-PT or PIN-PMN-PT), or piezocomposite made from piezoelectric materials, to have multiple thicknesses. Regions where the piezoelectric material is thicker will have a lower frequency response and regions where the piezoelectric material is thinner will have a higher frequency response. Machining a piezoelectric material to have different frequency responses will result in an ultrasound transducer with a larger overall bandwidth. However, piezoelectric materials, such as lead zirconate titanate (PZT) or piezoelectric single crystal are difficult and expensive to manufacture with multiple different thicknesses at the tolerances required in an ultrasound transducer.
Therefore, for these and other reasons, there is a need for an improved ultrasound transducer and ultrasound imaging system with improved bandwidth.